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I just figured out why I was having soooooo much trouble with my KT120 amp, so I wanted to share my saga. I had inadvertently misconnected the LM317HV pins. The current regulator page shows the out pin and the adjust pin connected together and then going through the "load". In actuality, the "adj" pin goes directly to ground, and the adjusting resistor goes from the "out" terminal to ground like the KT120 schematic. Of course I was too much in a fog to notice this little error, and practically scratched a hole in my skull trying to figure out just why I couldn't get the thing to work properly. Just for good measure (and because they're cheap) I paralleled 3 of the LM317HVT chips, so in theory they'll run cooler. So far, so good!

Hi, That will do it for sure. I parallel two in the KT120 amps. One would hold it with a really good heat sink, but the price of the heat sink is more than a second 317. I was initially worried that they would not share the current well, but my testing revealed that if they are from the same production batch they do very well. Under 10% differences and usually only about 5%. Cheap insurance.

Hello,I am looking at building a pair of Oddblocks and have a few questions. First is that I have the newest revision of the schematics; I found a power supply schematic dated May 19, 2013 which is marked up with blue writing, and an Oddblock Octal power amp drawing dated June 5, 2018. Can someone tell me if these are the most recent?For the power transformer, if I use the EMO813 spec’d for the KT120 tubes, but later settled on KT88 tubes, does that cause any issues? Also There was some discussion about including a sub signal out. If it doesn’t hurt anything to have it and not use it, I would plan for it in the layout. If someone would verify my interpretation of the discussion I would appreciate it. Thank you.

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Hi, Use the Octal version as it is the most current. This amplifier has evolved over the 8 + years it has existed. If you use the KT120 transformers (both power and output) you can use KT88s in the amps as well. All that is required is to use the lower current setting (about 90ma per tube vs 120-130 man per tube). The output impedance of the transformer is sufficient for this use, but not as perfect a match. It matches the KT120s but is about 25% low for the KT88s. You will lose a bit of power (2-4 watts) and the distortion level will be slightly higher (well below 0.5% change and not audible though) . If you don't need the 40-45 watts the KT88 version at about 25 watts is quite capable and is the version I use. There is an interesting twist to this. I have found, and others have verified that if you use KT120s in the amps designed for the KT88s and set it for the lower power setting in them (like for KT77s) to roughly 62 ma per tube you get about 20 watts of output that is really sweet. It is the best sounding combination I have ever found (others doing this agree). The measurements don't indicate a significant difference though. I suspect it has something to do with the construction of the tubes. One diyer said "big bottles just sound better", OK maybe. I use four of the amps set that way in my bi-amped set up and the sound is wonderful IMO. Considering I have all the others here to swap around, that is a clear indication of what I think of the combination. Next best is running the KT120 amps at the 120-130 ma level. Quite nice, but the other lower power combination is better. BTW running the KT120s in the amps designed for them at lower power does not achieve the same results. I attached the schematic of the PS I use. Yes you can use the sub woofer attachment like indicated. You can juggle the 4.7K and 220K a bit to get more output there. Be aware though that if you go to far it will upset the drive to the output tube. Personally with these amps I would use a powered sub and attach to the speaker terminals for the signal. The amps easily reach below 20HZ flat. Typical -3db point is about 10-12HZ. Plenty low enough for a sub amp input.

EDIT: I see it might be unclear that the wires from the LR8 do not connect to the box around that part of the circuit. Just pretend the box is not there. Also ignore the comment near the bottom on capacitor types after the LR8. I snached the schematic from some of my build plans and not from the kit amps as perhaps I should have done.

Good listeningBruce

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Unified OddBlock PS March 13, 2012b.jpg

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Hi Folks- Currently using two OddWatt-family amps on a daily basis. They're great!One minor issue is bias adjustment. I find that in particular with KT88s I need to adjustthe bias pretty regularly, and with a meter and pin jacks it is a bit of a pain.

I have dreamed up a microcontroller-based gadget to assist, which would displaythe bias current in the four power tubes simultaneously in a convenient way.

There are some minor complications: 40V DC potential on the current sense resistors,the need to ground the device (for safety), not wanting to inject noise during listening, etc.I have cooked up a draft schematic with 1M/1M voltage dividers on the inputs, instrumentationamps (INA128) and an Atmel ATMega328 uC. I've cooked up a draft schematic fora board which would drive an LCD character display. It would need 24V and 5V, whichwould come from some external switching supply.

Has anyone else done something like this? To be clear, I don't want to mount thisthing permanently in the amp, and would only plug it in periodically for a "tune-up".

Maybe this belongs as a new topic in the forum? Feel free to suggest that...

Hi, Clever solution...but most KT88s only need to be checked about every 6 months. It is likely a brand related thing as in the early days we found some brands that did not always do well in the amps. EH and Psvane were inconsistent in how they behaved. I suspect a thermal issue, but with so many other tubes that did work I never bothered to find out why. I measure and set my balance with two inexpensive digital panel meters in a plastic case with two 9 volt batteries. The batteries last forever as you only switch them on when using the device. Cheap and accurate.

Good listeningBruce

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Balance meter.jpg

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Yes, I thought about that, though I'm a bit nervous to turn over control of the bias to a microcontroller!Also, since this gadget is likely to be powered by a switching converter and contains digital logic, I am a bit concerned about injecting audible noise into the amp, so I was thinking of using it as a measurement tool only.

What currently-available brands of KT88 seem to work well? Currently I have Shuguang and have had a lot of trouble with them, so I put back in some JJ KT77s and for most listening they are completely fine, but I'd like to go back to KT88s for a bit more punch. Trying not to spend too much, was thinking of JJs or Mullard.

Hi, JJs work great and I supply them with the kits. Nice sound and trouble free. If you can get by with the power level of the KT77s, then you might consider Tung Sol KT120s running at the low power setting for KT77s. IMO they sound lovely. Output is about 19 wrms. You can run them as KT88s, but some of the magic is gone. They balance and stay within about 2-3% for months. No need to do anything in between. The amps don't need exact balance and mis-match as much as 5% makes no difference at 10% you lose a little output power. Others have reported issues with maintaining balance with Asian tubes. I suspect they are more sensitive to thermal changes and it causes them to shift the bias points slightly. If you go with JJ....the blue glass KT88s look neat.

I avoided auto bias as it added complexity to a diy project and really wasn't needed. There is as others have noted possible issues with introducing noise, pumping and other weird things. Simple works quite well and maintains the signal purity the best IMO.

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